Two independent openings of the BBB were demonstrated following one hour MCA occlusion in cats. The first opening occurred shortly after release of occlusion and was associated with high evaluations of the rCBF. The second opening of the barrier was demonstrable after 5 hours following release of occlusion and was associated with severe ischemic tissue changes. Both openings of the barrier were dependent on the rCBF falling below threshold values (12 m1/100 g/min) during the occlusion. The first opening of the barrier is considered as being due to hemodynamic disturbances produced by reactive hyperemia in areas devoid of autoregulation. The second barrier opening is considered to occur due to some factors derived from severly injured ischemic brain tissue.